1. Technical Field
The present invention relates to a multilayer type power inductor, and more particularly, to a multilayer type power inductor having improved temperature characteristics by including a gap layer having an asymmetrical structure.
2. Description of the Related Art
A multilayer type power inductor is mainly used in a power supply circuit such as a DC-DC converter within a portable device. A multilayer type power inductor having a small size, a high current, a low DC resistance, or the like, has been mainly developed. In accordance with the trend for a DC-DC converter having a high frequency and a small size, the use of the multilayer type power inductor instead of the existing wire-wound type choke coil according to the related art has increased.
In the case of the multilayer type power inductor, magnetic saturation of the inductor is materially/structurally suppressed, such that the inductor may be used at a high current. The multilayer type power inductor has a disadvantage in that a change in inductance (L) value according to the application of current thereto is larger; however, has an advantage in that it has a smaller size and a thinner thickness and is also advantageous in terms of a DC resistance, as compared to the wire-wound type power inductor (See FIG. 1).
A power inductor having a small change in inductance value with respect to a used current has been required. Particularly, a power inductor capable of being operated from a low temperature of −55° C. to a high temperature of +125° C. and having a small change in inductance value with respect to a temperature has been increasingly required.
Particularly, the wire-wound type power inductor has a small change in inductance (L) value according to the application of current thereto. Also in the multilayer type power inductor, an effort for implementing the small change in inductance (L) value according to the application of current has been conducted. For this, it has been shown that factors such as a composition of a material, a micro-structure, a structural design, or the like, are important. In other words, the multilayer type power inductor has a disadvantage in that a change in inductance (L) values according to the application of current thereto is larger, as compared to the wire-wound type power inductor. This is the reason that the wire-wound type power inductor has structurally a larger open magnetic path effect.
Therefore, in the multilayer type power inductor, it is important to improve change characteristics in inductance (L) value according to the application of current. Currently, a gap layer has been partially included in an inner structure of the multilayer type power inductor to cut a magnetic flux, thereby improving the change characteristics in inductance (L) value according to the application of current. On the other hand, the multilayer type power inductor has a simple structure, a small size, and a thin thickness, and is advantageous in securing price competitiveness.
A structure of a general multilayer type power inductor that is currently being used is shown in FIG. 2. Referring to FIG. 2, internal electrodes 10 are formed and a gap layer 30 is inserted into a body 20 made of a ferrite material to block a magnetic flux, thereby decreasing a change in inductance value according to the application of current. Then, firing is performed at a temperature of about 900° C., external electrodes 40 are formed, and plating is then performed, such that a plating layer 50 is formed.
However, this multilayer type power inductor has a disadvantage in that a change in inductance value according to the application of current thereto is large according to a change in temperature, such that temperature stability is low. This is the reason that temperature characteristics change due to diffusion according to a temperature because copper (Cu)-substituted Zn-ferrite is used as a non-magnetic material, which is a material of the gap layer.
The basic concept of design for the multilayer type power inductor is that even though the efficiency of a coil decreases, change characteristics in inductance value according to the application of current (hereinafter, DC bias characteristics) is allowed to be improved, such that a change in inductance (L) value according to the application of current is maximally suppressed.
The smaller the change in inductance (L) value the application of current is, the more excellent the DC bias characteristics become. The lower the inductance (L) value is, the larger the ripple of an output voltage becomes and the lower the efficiency becomes. The lower a DC resistance is, the higher the efficiency becomes. Particularly, the efficiency becomes high at a high current. Changes in inductance (L) value according to the application of current at each temperature are measured. In this case, it is preferable that the change in inductance (L) value according to the application of current at each temperature is small.
The DC-bias characteristics of a chip inductor are a function of characteristics of a material and a coil structure. First, in the case of materials having the same-magnetic permeability, the higher the saturation magnetization (Ms) of the material is, the more excellent the DC-bias characteristics may become. Therefore, basically, a material having excellent DC-bias characteristics need to be selected at the time of selection of a composition. There is also a need to consider a grain size. Generally, the DC-bias characteristics are excellent at a small grain size. Since density of a material itself and density of an electronic spin are in proportion to each other, it is also necessary to decrease pores of the material in order to improve the DC-bias characteristics.
Meanwhile, the DC-bias characteristics of the material may change according to a magnetic permeability. That is, the lower the magnetic permeability is, the more excellent the DC-bias characteristics may become. However, the turn number of the coil need to be increased in order to implement the same inductance. In this case, since a magnetic flux flowing in the coil increases, an effect in which the magnetic saturation of the material is delayed decreases by half.
Whether or not there is really a merit in this case may be predicted from a magnetic circuit equation. For convenience, it is assumed that a change rate in material according to the magnetic saturation corresponds to a function of a magnetic flux. The following Equation 1 is obtained from the magnetic circuit equation.
                    L        =                              N            2                                R            f                                              [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, L indicates an inductance value, N indicates the turn number of the coil, and Rf indicates a resistance value of ferrite.
Since Rf increases when the magnetic permeability is different, capacitance need to be adjusted by increasing the turn number (N).
When magnetic resistance values in a high magnetic permeability coil (Coil 1) in a structure shown in FIG. 3A and a low magnetic permeability coil (Coil 2) in a structure shown in FIG. 3B are expressed as R1 and R2 and variables according to a change in structure are expressed as N1 and N2, the following Equation 2 is obtained.
                    L        =                                            N              1              2                                      R              1                                =                                                                      N                  2                  2                                                  R                  2                                            →                              R                2                                      =                                                            (                                                            N                      2                                                              N                      1                                                        )                                2                            ⁢                              R                1                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
On the other hand, a magnetic flux flowing in a material itself is a function of the turn number and the magnetic resistance value. Therefore, magnitudes of the magnetic fluxes in each of the structures may be compared with each/other from the following Equation 3.
                                          ϕ            1                    =                      NI                          R              1                                      ⁢                                  ⁢                              ϕ            2                    =                                                                      N                  2                                ⁢                I                                            R                2                                      =                                                                                                      N                      2                                        ⁢                    I                                                        R                    1                                                  ⁢                                                      (                                                                  N                        1                                                                    N                        2                                                              )                                    2                                            =                                                                                                                  N                        1                                            ⁢                      I                                                              R                      1                                                        ·                                      (                                                                  N                        1                                                                    N                        2                                                              )                                                  =                                                      ϕ                    1                                    ·                                      (                                                                  N                        1                                                                    N                        2                                                              )                                                                                                          [                  Equation          ⁢                                          ⁢          3                ]            
Since N1<N2, the magnetic flux flowing in the Coil 2 is actually smaller than the magnetic flux flowing in the Coil 1. Therefore, it is predicted that a change rate in magnetic permeability will be smaller and DC-bias characteristics will be more excellent in the Coil 2 than in the Coil 1.
In a multilayer type power inductor having a gap layer inserted thereinto as shown in FIG. 4, an effect of the gap layer will be described. When a magnetic material structure of a magnetic circuit is cut by a non-magnetic material or an air gap, a magnetic resistance increases, such that a magnitude of a magnetic flux flowing in the magnetic circuit decreases. Therefore, an effective magnetic permeability decreases, and an inductance decreases accordingly. However, a change in inductance (L) value becomes significantly small. This influence is expressed by the following Equation 4.
                                          Δ            ⁢                                                  ⁢                          L              e                                            L            e                          ≈                                            Δ              ⁢                                                          ⁢                              μ                r                                                    μ              r                                ⁢                                    (                                                μ                  e                                                  μ                  r                                            )                        2                                              [                  Equation          ⁢                                          ⁢          4                ]            
Therefore, when the effective magnetic permeability decreases by the gap layer made of the non-magnetic material, the DC-bias characteristics are improved by square of the effective magnetic permeability.
When there is the gap layer, the inductance may be expressed by the following Equation 5.
                    L        =                              N            2                                              R              g                        +                          R              f                                                          [                  Equation          ⁢                                          ⁢          5                ]            
In Equation 5, Rg indicates a magnetic resistance of the gap layer and Rf indicates a magnetic resistance of the ferrite.
Here, when the coil is perfectly designed and a cross sectional area in a magnetic flux path is constant, a relationship between a magnetic permeability and a magnetic resistance of the ferrite may be expressed by the following Equation 6 and Equation 7.
                              R          f                =                                            l              e                                                      μ                r                            ⁢                              μ                o                            ⁢                              S                e                                              =                      A                          μ              r                                                          [                  Equation          ⁢                                          ⁢          6                ]                                L        =                              N            2                                              R              g                        +                          A              /                              μ                r                                                                        [                  Equation          ⁢                                          ⁢          7                ]            
In Equation 6 and Equation 7, le indicates an effective path of the magnetic flux, Se indicates an effective cross sectional area of the magnetic flux, and A is a constant.
Therefore, in the case of the general inductor, a change in inductance value is in direct proportion to the magnetic permeability; however, in the case of the inductor including the gap layer, Rg is significantly larger than Rf, such that a change in magnetic permeability does not significantly have an influence on the inductance.
As described above in detail, the power inductor has the gap layer inserted thereinto, such that the DC-bias characteristics of the power inductor may be significantly improved.
However, when the power inductor is actually used, the DC-bias characteristics according to a change in temperature (hereinafter, referred to as Bias-TCL) as well as the DC-bias characteristics at a room temperature need to be excellent.
FIG. 5A shows a case in which a change in inductance value is significantly small after currents measured at each temperature are applied; and FIG. 5B shows a case in which bias-TCL characteristics according to a temperature are deteriorated. When the power inductor has the deteriorated DC-bias characteristics as shown in a graph of FIG. 5B, it is difficult to use the power inductor in a DC-DC converter.
The bias-TCL according to a temperature is in correlation with a kind of material used in the gap layer. As a material of the existing gap layer, ZnCu-ferrite in which ZnO is substituted with a small amount of CuO in Zn-ferrite (ZnFe2O4) is used. Since the material of the gap layer is the non-magnetic material, it is preferable that ferrite having a significantly low Curie temperature to thereby have non-magnetism at a room temperature is appropriate to be used as the material of the gap layer. For example, the Zn-ferrite (ZnFe2O4) is appropriate to be used as the material of the gap layer because it has a significantly low Curie temperature of 35 K or less.
However, there is a disadvantage in that it is difficult to sinter the Zn-ferrite at a temperature of 900° C. or less. Generally, in the multilayer type power inductor, silver (Ag) is used as a material of an internal electrode. Since the silver has a melting point of 961° C., sintering need to be performed at a temperature of about 900° C. However, the Zn-ferrite is not well sintered at the temperature of about 900° C. Therefore, in order to improve sinterability, ZnO is substituted with a small amount of CuO in Zn-ferrite (ZnFe2O4) in the Zn-ferrite, such that the sintering may be performed at the temperature of about 900° C.
In addition, since ZnCu ferrite has a spinel structure in which it does not have a lattice mismatch with NiZnCu ferrite used as a material of the body, it may decrease delamination that may be generated at the time of sintering of the multilayer type power inductor.
However, the ZnCu ferrite is not a complete non-magnetic material, has a Curie temperature of a room temperature or less, and shows non-magnetic material characteristics at the room temperature. However, a thickness of the non magnetic material decreases due to the diffusion of Ni and Cu at the time of firing (See FIG. 6).
In addition, as shown in FIG. 6, Ni is diffused to the gap layer and enters the gap layer at the time of firing and positions to which Ni is diffused have magnetism, such that the entire thickness of the gap layer made of the non-magnetic material decreases. The decrease in thickness of the gap layer is generated because positions having different Curie temperatures according to a temperature are generated, such that a thickness of the gap layer made of the non-magnetic material according to the temperature changes as shown in FIG. 7.
When the thickness of the gap layer made of the non-magnetic material increases, the DC-bias characteristics are improved, and when it decreases, the DC-bias characteristics are deteriorated. Therefore, in order to use the ferrite non-magnetic material, a gap layer capable of suppressing this diffusion is needed. Mutual diffusion between the magnetic material ferrite of the body and the non-magnetic material ferrite is generated, thereby making it possible to deteriorate characteristics of the power inductor.
Meanwhile, the power inductor according to the related art is the multilayer type power inductor having a structure as shown in FIG. 4 and is made of a ferrite sheet. Here, NiZnCu ferrite having ferrimagnetism is used as a material of the body.
Non-magnetic material ferrite (generally, ZnCu ferrite) having ferrimagnetism is used in the entire surface sheet gap or an open sheet gap as a material of the gap layer. Firing is performed at a temperature of about 900° C., external electrodes are formed, and plating is then performed.
However, the multilayer type power inductor according to the related art in which the gap layer is made of the ZnCu ferrite has the following problems.
(1) An Ni component contained in the NiZnCu ferrite, which is a material of the body, is diffused into the gap layer and a Zn component of the gap layer is diffused into the body, such that a thickness of the gap layer made of the non-magnetic material decreases. When the thickness of the gap layer made of the non-magnetic material decreases, the DC-bias characteristics may be deteriorated.
Therefore, since the thickness of the gap layer made of the non-magnetic material need to be increased in order to improve the DC-bias characteristics, a gap sheet inserted before sintering need to have a thick thickness. However, when the gap sheet having the thick thickness is used, a thickness (at direction) of the multilayer type power inductor increases.
(2) A predetermined level of magnetic flux is blocked; however, there is a risk of delamination due to a difference in contraction percentage between the ZnCu ferrite and the ferrite material of the body at the time of sintering and stress may be generated in an inner portion of the power inductor.
(3) The bias-TCL characteristics are deteriorated due to the diffusion of the gap layer.